1. Field of the Invention
The present invention relates generally to lubrication systems for internal combustion engines and, more particularly, to an arrangement for filtering the oil in an engine lubricating system wherein the flow circuit including the filter is separate from the lubrication flow circuit of the system.
2. Description of the Prior Art
Oil lubrication systems for lubricating the working components of an internal combustion engine are most effective and the life of the oil is extended as long as the oil remains clean. However, it is to be expected, that the oil will pick up impurities such as metal and dirt particles as the oil circulates about the lubricating system. This is especially true in dusty environments in which many small engines are used such as, for instance, in lawn mowers and the like. If such impurities are permitted to remain in the oil they will cause premature wear of the working components of the engine. Furthermore if such impurities remain in the oil they will tend to clog the circulation flow passages of the lubrication system, thereby preventing sufficient oil from reaching the working components of the engine, and contributing further to engine wear.
Thus, in order to avoid premature wear of the working components of the engine, it is necessary to remove these impurities or otherwise change the oil frequently. Since changing the oil in engines is time consuming, it is desirable that an oil filtering system is provided. Prior art engines have been provided which include systems for removing impurities from lubricating oil. Both oil separating and filtering techniques have been provided in such prior art arrangements. Examples of oil separating systems are disclosed in U.S. Pat. No. 3,078,958 and U.S. Pat. No. 1,986,539. A disadvantage of such separating systems is the relative complexity of the systems and the need to periodically clean the oil separators to remove the impurities therefrom. Furthermore, such systems are not as effective to remove impurities from the oil as are filtering systems because they depend upon the centrifugal separation of impurities from the oil rather than on removing the impurities with a positive filtering barrier or oil filter element in the filtering circuit for positively preventing impurities from remaining entrained in the circulating oil. It is therefore desirable that lubricating oil is filtered to remove the impurities therein.
Small internal combustion engines, such as used on lawn mowers and other small tools and implements, are generally splash or pressure spray lubricated. Such splash or pressure spray lubrication systems are not compatible with a filtration system arranged in series with the lubrication system because, during normal operation, insufficient or no oil pressure is available for operating a satisfactory oil filtering system and lubricating the engine. In general, the lubrication pumps used in those systems are positive displacement pumps such as plunger or gyrator type of pumps which, on start-up at lower temperatures with high oil viscosities, will cause high oil pressure to be generated which will cause the oil filter element to fail unless a pressure relief valve is added to the system.
An additional problem encountered in small engines is that the oil filters used therein tend to be relatively small, thereby clogging rather quickly and preventing oil flow from lubricating the engine if the filter is located in the main lubrication flow circuit. Accordingly many small engines have been provided without any oil filtration system, thereby causing excessive engine wear and resulting in the need for frequent oil changes. It is therefore desired to provide a lubrication system for a small engine including an oil filtering circuit which is economical to manufacture and install in small internal combustion engines.